The majority of precast, prestressed, hollow-core slabs are manufactured in either a wet casting process or by extrusion casting. In a typical wet casting process a concrete mix having a slump of from two inches to three inches is poured into a concrete slab form around inflatable, core-forming tubes and prestressing wires held in place in the form, the form vibrated and then placed in a kiln for curing of the concrete. While wet casting provides an excellent structural slab the equipment capital expense of a wet casting facility may be significantly greater than the cost of an extrusion casting line.
In an extrusion casting operation a relatively dry concrete mix is used because the hollow core slab must be essentially self-supporting immediately after extrusion. Therefore a dry mix with less than a one inch slump is generally used even though this is known to be a cause of many problems normally associated with extrusion casting.
For example, a concrete sufficiently dry to be immediately self-supporting may not feed consistently through the extruder, resulting in areas of reduced pressure, surface and internal cavities and inadequate bond between the concrete and prestressing steel.
Thus concrete in a relatively dry mix, even though vibrated, does not act as a true fluid with a continuous pressure throughout the confined area. Friction between particles and between the particles and confinement means results in rapid pressure transfer losses. As a result there may be a variance of pressure within the body of concrete.
Prior art extrusion apparatus usually places the core forming augers beneath the feed opening into the extruder with the outside diameters of the augers approximately the same as the inside diameter of the core in the finished slab. With this construction the turning augers hinder or otherwise restrict the flow of concrete onto the casting bed beneath the augers. Any concrete that falls into this area must do so through the openings between augers or by falling between flights of the augers as the augers turn.
This manner of filling the area beneath the augers often leaves a void resulting in cavitation and a variance of pressure in the mass of concrete. Aside from cavitation problems the variance of pressure along with a movement of the concrete in an attempt to equalize the pressure, will tend to displace reinforcing steel from the desired position within the apparatus.
Thus, despite the advantages of generally lower capital cost prior art extrusion casting systems are subject to the disadvantages of internal and surface cavitation problems, improper bonding of concrete to steel reinforcing and porous or otherwise undesirable surface finishes.